Contribution of five amino acid residues in the glutathione-binding site to the function of human glutathione transferase P1-1

Abstract

Five amino acids in proximity to GSH bound in the active-site cavity of human Class Pi glutathione transferase (GST) P1-1 were mutated by oligonucleotide-directed site-specific mutagenesis. The following mutations gave catalytically active mutant proteins with the proper dimeric structure: Arg14----Ala, Lys45----Ala, Gln52----Ala, Gln65----His and Asp99----Asn. The mutation Gln65----Ala was also made, but the protein was not characterized because of its poor catalytic activity. Residues Arg14, Lys45, Gln52 and Gln65 all contribute to binding of glutathione, and the substitutions caused an approx. 10-fold decrease in affinity, corresponding to 5 kJ/mol, except for Arg14, for which the effect was larger. In addition, Arg14 appears to have an important structure role, since the Arg14----Ala mutant demonstrated a significantly lower stability as compared with the wild-type and the other mutant enzymes. Asp99 primarily contributes to catalysis rather than to binding. The kcat./Km-versus-pH profile for the Asp99----Asn mutant is shifted by 0.5 pH unit in the alkaline direction, and it is proposed that Asp99 may participate in proton transfer in the catalytic mechanism. The possibility of redesigning the substrate specificity for GSTs was shown by the fact that the mutant Lys45----Ala displayed a higher catalytic efficiency with GSH monoethyl ester than with its natural substrate, GSH.